The present invention concerns ceilometric methods and apparatuses of the type in which a transmitter emits a pulsed beam of light, and the pulsed beam directed upwards and reflected back down from one or more overhead cloud layers is downwardly incident upon a photodetector. Transmission of the photodetector output signal is gated in accordance with range-gating technique. The start and end of the gating interval, measured relative to the time of emission of a light pulse from the transmitter, is correlated with the round-trip travel time for the emitted radiation from the transmitter to an overhead cloud, if any, located at a predetermined ceilometric range step. The time of occurrence of the gating interval, relative to the pulse emission instant, is stepwise shifted, so as to establish a succession of range steps each associated with a respective potential cloud elevation. Before going on from one range step to the next, the time interval spent at one range step, i.e., using one particular gating interval, is equal to a multiple of the period of the emitted light pulses, so that a plurality of light integrations be performed at each range step before proceeding onto the next one.
Such methods and apparatuses are known, for example, from Federal Republic of Germany published allowed patent application (Auslegeschrift) No. 2,150,969. The integrated photodetector signal developed during the repeated light measurements performed during the dwell at each individual range step is recorded by means of a pen recorder instrument. The drive mechanism for the pen of the recorder is coupled with a potentiometer which is repeatedly adjusted for effecting the successive shifts with respect to emission time of the time of occurrence of the gating interval, so as to effect the runthrough through the succession of range steps.
The operation of the recording system, being dependent upon the shifting with respect to time of the time of occurrence of the gating interval, necessitates the use of a relatively costly and somewhat malfunction-prone mechanism. Furthermore, because of the type of recording technique employed, the time required to record, as opposed to that required to perform, one complete runthrough through the whole succession of range steps is relatively lengthy. Problems therefore arise when such recording technique is to be used in conjunction with flight-safety ceilometers which, in accordance with international aviation regulations, must repeat a runthrough through the entire ceilometric range explored at least once every 15 seconds.